You can certainly purchase the travel insurance right away. You will probably want to include the cost of your airline tickets in the trip cost. If you can find out what the price of the tickets will be before purchasing them, you may include that amount in the total trip cost. If not, an estimate will have to do. Otherwise, buy the policy now with the costs expended to date and modify the policy when you make your other arrangements.If there is any additional premium as a result of the change, it will be charged at that time. Be sure you do this within the 7 – 21 day purchase requirement (depends on company) in order to comply with the pre-existing condition waiver requirements.